In solid-state physics, the work function defines the minimum energy required to remove an electron from a solid to a point immediately outside the surface of the solid. In other words, the work function is the amount of energy needed to move the electron from the highest filled Fermi level into the vacuum immediately outside the solid surface. This amount of energy is typically measured in electron volts, and as opposed to being a property of a bulk material itself, the work function is a characteristic property for a surface of the material.